1. Field of the Invention
The invention relates to a new process for preparing methacrolein (2-methylpropenal) from propionaldehyde and formaldehyde.
2. Discussion of the Prior Art
Methacrolein is an important intermediate in the preparation of various organic compounds, in particular in the fields of perfumes and fragrances, pharmaceuticals, and polymers.
Numerous methods are already known for preparing methacrolein. Thus, for example, methacrolein can be obtained from isobutene by catalytic oxidation. The olefin is passed at 300.degree. to 600.degree. C. over suitable catalysts, in particular those containing vanadium, tungsten or molybdenum. The yields are between 40 and 80 percent referred to converted isobutylene. The use of high temperatures and sensitive catalysts raises a number of chemical engineering and technical problems that restrict the applicability of this method of preparation.
Reference may also be made to the preparation of methacrolein by oxidizing isobutyraldehyde on a catalyst containing molybdenum or uranium. (See French No. 13 40 385). The required reaction temperature is 275.degree. C. to 375.degree. C. The disadvantage of this process is the low yield, which is only about 25 percent.
A further group of processes for preparing methacrolein start from propionaldehyde and formaldehyde. One such process is based on the condensation of the starting aldehydes at 275.degree. C. using catalysts containing sodium oxide and silicic acid. Methacrolein is obtained in a yield of about 46 percent by this process (sec. C.A. Vol. 56 (1962) 2321 and 2322).
Methacrolein can be prepared from propionaldehyde by means of a Mannich reaction, i.e., the condensation of ammonia or a primary or secondary amine, normally present as a salt, with formaldehyde and a compound containing a reactive hydrogen atom. Corresponding processes are described in C.A. Vol. 59 (1963), 393, 394, and also in U.S. Patent Specifications 144,164 and 2,848,499.
Despite the good methacrolein yields that can be obtained by the Mannich reaction, this process is not widely used industrially. A drawback is the necessity to use amines in stoichiometric amounts. In addition, the use of hydrochlorides, in which form the amines are generally employed, has considerable disadvantages. Thus, halogen-resistant apparatus must be used in order to prevent the occurrence of stress crack corrosion, which is a danger particularly when operating under elevated pressure.
Attempts have been made to circumvent these difficulties by carrying out the reaction between propionaldehyde and formaldehyde in the liquid phase using sulphuric acid or p-toluenesulphonic acid as catalysts (see Japanese No. 23 159). However, this reaction produces methacrolein in a yield of only about 60 percent.
To summarize, it may be said that the heretofore known processes for preparing methacrolein have considerable disadvantages, particularly as regards the low yields or difficulties experienced in the technical implementation.
It therefore became desirable to provide a process that avoids the afore-mentioned disadvantages and produces methacrolein in high yields without great technical expense or effort.